ITO (indium tin oxide) is a compound produced by adding a few % of tin oxide (SnO2) to indium oxide (In2O3); and since it is electroconductive and is highly transparent as having a visible light transmittance of about 90% or so, it is used as an electrode mainly for flat panel displays (FPD); and with the recent increase in the shipment of FPD, the demand for ITO transparent electroconductive thin films is expanding.
However, indium that is the main ingredient of ITO is a rare metal and the exhaustion of indium resources is a serious problem; and the sense of crisis about it is increasing and the indium cost is increasing.
Accordingly, methods of collecting ITO wastes for recycling indium have been proposed, and further, trials of increasing the collection rate have been tried; however, as a radical resolution, development of materials substitutive for ITO transparent electroconductive thin films is greatly desired.
As a material substitutive for ITO transparent electroconductive thin films, proposed is a transparent electroconductive thin film of carbon nanotubes (see Patent Reference 1). This Patent Reference 1 discloses a technique of disposing carbon nanotubes on a transparent substrate as dispersed thereon, thereby providing a 550-nm light transmittance of 95% and a surface resistivity of from 105 to 1011Ω/sq.
Of carbon nanotubes, however, single-walled carbon nanotubes (SWNTs) include metallic ones (m-SWNTs) and semiconductor ones (s-SWNTs) inevitably as mixed therein in their production process; but in conventional thin films of SWNTs, nothing is taken into consideration about the mixed m-SWNTs and s-SWNTs. Accordingly, the compatibility between the electroconductivity and the light transmittance of thin films is limited.
In conventional thin film formation techniques with SWNTs, a polymer such as an acidic polymer of an alkylammonium salt, a polyoxyethylene-polyoxypropylene copolymer or the like is used as the dispersant for SWNTs, and therefore the thin films are characterized as SWNTs-containing polymer thin films; and the same situation applies to the case of Patent Reference 1. In such thin films, the polymer dispersant remains, and therefore, some limitations are given to the compatibility between the electroconductivity and the light transmittance of the thin films and to the process of forming the thin films.
The present inventors are promoting studies of dispersing single-walled carbon nanotubes with an amine as a dispersant; and in the past, the inventors have proposed a technique of concentrating SWNTs as combined with centrifugation or the like (see Patent Reference 2), but have heretofore made no investigations about thin film formation using them and about the physical properties such as light transmittance, electroconductivity and the like of the thin films, and any concrete facts have not been clarified at all.    Patent Reference 1: JP-A-2006-049843    Patent Reference 2: WO2006/013788